Methods and apparatus for generating a library of spectra

ABSTRACT

A method of generating a library from a reference substrate for use in processing product wafers is described. The method includes measuring substrate characteristics at a plurality of well-defined points of a reference substrate, measuring spectra at plurality of measurement points of the reference substrate, there being more measurement points than well-defined points, and associating measured spectra with measured substrate characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC §119(e) to U.S. patentapplication Ser. No. 12/059,435, filed on Mar. 31, 2008, which claimsthe benefit of prior U.S. Provisional Application 60/949,498, filed Jul.12, 2007, and U.S. Provisional Application 60/909,639, filed Apr. 2,2007 the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to metrology, and in one aspect to opticalmonitoring of substrates during a chemical mechanical polishing process.

BACKGROUND

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive, or insulativelayers on a silicon wafer. One fabrication step involves depositing afiller layer over a non-planar surface and planarizing the filler layer.For certain applications, the filler layer is planarized until the topsurface of a patterned layer is exposed. A conductive filler layer, forexample, can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. After planarization, theportions of the conductive layer remaining between the raised pattern ofthe insulative layer form vias, plugs, and lines that provide conductivepaths between thin film circuits on the substrate. For otherapplications, such as oxide polishing, the filler layer is planarizeduntil a predetermined thickness is left over the non planar surface. Inaddition, planarization of the substrate surface is usually required forphotolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is placed against a rotating polishing pad. Thepolishing pad may be either a “standard” pad or a fixed-abrasive pad.The carrier head provides a controllable load, i.e., pressure, on thesubstrate to push it against the polishing pad. A polishing liquid, suchas a slurry with abrasive particles, is supplied to the surface of thepolishing pad.

In order to determine the effectiveness of a polishing operation, a“blank” substrate (e.g., a wafer with multiple layers but no pattern) ora test substrate (e.g., a wafer with the pattern to be used for devicewafers) is polished in a tool/process qualification step. Afterpolishing, the substrate is removed from the polishing system and theremaining layer thickness (or another substrate property relevant tocircuit operation, such as conductivity) is measured at several pointson the substrate surface using an in-line or stand-alone metrologystation. The variation in layer thickness provide a measure of the wafersurface uniformity, and a measure of the relative polishing rates indifferent regions of the substrate. The in-line or stand-alone metrologystation can provide extremely accurate and reliable thicknessmeasurements (e.g., using ellipsometry) and precise positioning of asensor to desired measurement locations on the substrate. However, thismetrology process can be time-consuming, and the metrology equipment canbe costly.

One problem in CMP is determining whether the polishing process iscomplete (i.e., whether a substrate layer has been planarized to adesired flatness or thickness). Variations in the initial thickness ofthe substrate layer, the slurry composition, the polishing padcondition, the relative speed between the polishing pad and thesubstrate, and the load on the substrate can cause variations in thematerial removal rate. These variations cause variations in the timeneeded to reach the polishing endpoint. Therefore, for someapplications, determining the polishing endpoint merely as a function ofpolishing time can lead to unacceptable variations in the post-polishingthickness of the substrate layer. However, removal of the substrate fromthe polishing apparatus for transportation to an in-line or stand-alonemetrology station can lead to an unacceptable reduction in throughput.

Several methods have been developed for in-situ polishing endpointdetection. One class of methods involve optically monitoring thesubstrate during polishing, e.g., using an optical sensor positioned inthe platen that directs a light beam through a window onto thesubstrate. However, measurements using such an in-situ system usuallycannot be precisely positioned at a desired measurement location due tothe motion of the substrate relative to the sensor, and the measurementscan be less accurate due to noise generated by the polishing environment(e.g., absorption of light by slurry), the limited time available formeasurements, and the need for real-time processing of the sensor data.

SUMMARY

This invention relates to a method of generating a library from areference substrate for use in processing product wafers. The methodincludes measuring substrate characteristics a plurality of well-definedpoints of a reference substrate, measuring spectra at plurality ofmeasurement points of the reference substrate, there being moremeasurement points than well-defined points, and associating measuredspectra with measured substrate characteristics.

Implementations of the invention may include one or more of thefollowing. Coordinates of the well-defined points and coordinates of themeasurement points may be stored. Associating measured spectra withmeasured substrate characteristics can include comparing coordinates ofthe well-defined points with coordinates of the measurement points.Comparing coordinates of the well-defined points with coordinates of themeasurement points can include determining a distance a spectra and awell-defined point.

Associating measured spectra with measured substrate characteristics caninclude determining a well-defined point that is nearest to a particularmeasurement point, and associating the substrate characteristic of thedetermined well-defined point with the spectra of the particularmeasurement point. The substrate characteristic can include a layerthickness, such as a pre- or post-polish layer thickness. Identicalspectra exhibiting different layer thickness values can be removed. Theplurality of well-defined points can be at substantially similarrelative locations within different dies on the reference substrate. Atleast some of the measurement points are spatially different than thewell-defined points. The substrate characteristics can be measured priorto or after measuring the spectra. Measuring the spectra can includescanning a sensor across the reference substrate. A method of monitoringa substrate can include generating a library from a reference substrateaccording to the method above, scanning a product substrate with aoptical monitoring system to generate a plurality of spectra, anddetermining substrate characteristics for the product substrate based onthe library. Scanning the product substrate can include scanning with anin-situ monitoring system or scanning with an in-line monitoring system.

In another aspect, a method of generating a library for use inprocessing product wafers includes measuring a substrate layer thicknessat a first well-defined point and a second well-defined point of areference substrate, measuring a spectra at a first measurement point ofthe reference substrate, determining the closer of the firstwell-defined point and the second well-defined point to the firstmeasurement point, and associating the spectra with the substrate layerthickness of the closer well-defined point.

In another aspect, a computer program product, tangibly stored onmachine readable medium, includes instructions operable to cause aprocessor to perform or cause the steps of the various methods above.

In another aspect, a substrate processing system includes a processingmodule to process a substrate, a factory interface module configured toaccommodate at least one cassette for holding the substrate, aspectrographic monitoring system positioned in or adjoining the factoryinterface module, and a substrate handler to transfer the substratebetween the at least one cassette, the spectrographic monitoring systemand the processing module.

Implementations of the invention may include one or more of thefollowing. The spectrographic monitoring system include may an opticalprobe and may be configured to measure spectra at a plurality ofpositions on the substrate while the substrate is moving relative to theoptical probe. The substrate may be moved by the substrate handler andthe optical probe may remain stationary. Spectra may be measured in aplurality of positions that span a diameter of the substrate in lessthan ten seconds. The plurality of positions may form a non-linear pathon the substrate, e.g., a figure-eight path. The spectrographicmonitoring system may include an optical probe and may be configured tomeasure spectra at a plurality of positions on the substrate withoutaligning the optical probe to well-defined locations on the substrate.The spectrographic monitoring system may be positioned in the factoryinterface module. A notch alignment system may position a notch of thesubstrate in a determined orientation.

As used in the instant specification, the term substrate can include,for example, a product substrate (e.g., which includes multiple memoryor processor dies), a test substrate, a bare substrate, and a gatingsubstrate. The substrate can be at various stages of integrated circuitfabrication, e.g., the substrate can be a bare wafer, or it can includeone or more deposited and/or patterned layers. The term substrate caninclude circular disks and rectangular sheets.

Possible advantages of implementations of the invention can include oneor more of the following. A library of spectra can be assembled, and thespectra can be associated with physical properties of the substrate.Spectra-based endpoint determination can be made in-situ with greaterspeed and accuracy, and variations in the post-polishing thickness ofthe substrate layer can be reduced. Spectra-based measurements ofsubstrate characteristics can be made by in-line monitoring systems withgreat speed.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional side view of an exemplary chemicalmechanical polishing apparatus having an in-situ optical monitoringsystem.

FIG. 2 illustrates an exemplary path of spectra measurements by anin-situ monitoring system across a substrate.

FIG. 3 shows an exemplary process for generating a library thatassociates substrate characteristics with spectra.

FIG. 4 illustrates a portion of a reference wafer having exemplarywell-defined points.

FIG. 5 illustrates a data structure associating a substratecharacteristic with a coordinate for each well-defined points.

FIG. 6 illustrates a portion of a reference wafer having exemplarymeasurement points.

FIG. 7 illustrates a data structure associating a spectrum with acoordinate for each measurement point.

FIG. 8 illustrates a library with a data structure associating spectrawith substrate characteristics.

FIG. 9 illustrates an exemplary method for associating spectra withsubstrate characteristics.

FIG. 10 illustrates an exemplary verification process for data stored ina library.

FIG. 11 shows a method for using spectrum based endpoint determinationto determine an endpoint of a polishing step.

FIG. 12 is a top view of an exemplary substrate processing system havingan in-line spectrographic monitoring system.

FIG. 13 is a perspective view of an interior of an exemplary factoryinterface module.

FIG. 14 is a side view of an exemplary factory interface module havingan in-line spectrographic monitoring system.

FIG. 15 illustrates an exemplary path of an optical probe of the in-linespectrographic monitoring system across a reference substrate duringspectrographic measurements for library generation.

FIG. 16 illustrates an exemplary path of an optical probe of the in-linespectrographic monitoring system across a device substrate duringspectrographic measurements for data collection for processing control.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1, one or more substrates 10 will be polished at apolishing station of a chemical mechanical polishing (CMP) apparatus 20.A description of a polishing apparatus can be found in U.S. Pat. No.5,738,574, the entire disclosure of which is incorporated herein byreference.

The polishing station includes a rotatable platen 24 on which is placeda polishing pad 30. The platen 24 can be connected to a platen drivemotor (not shown). For most polishing processes, the platen drive motorrotates platen 24 at thirty to two hundred revolutions per minute,although lower or higher rotational speeds may be used. The polishingstation can also include a pad conditioner apparatus to maintain thecondition of the polishing pad.

Polishing pad 30 typically has a backing layer 32 which abuts thesurface of platen 24 and a covering layer 34 which is used to polish thewafer 10. Covering layer 34 is typically harder than backing layer 32.However, some pads have only a covering layer and no backing layer.Covering layer 34 can be composed of a polyurethane with pores, e.g., afoamed polyurethane or cast polyurethane with microspheres, and agrooved surface. Backing layer 32 can be composed of compressed feltfibers leached with urethane. A two-layer polishing pad, with thecovering layer composed of IC-1000 and the backing layer composed ofSUBA-4, is available from Rodel, Inc., of Newark, Del. (IC-1000 andSUBA-4 are product names of Rodel, Inc.).

A carrier head 80 can be supported by a rotatable multi-head carousel.Generally, the carrier head holds the wafer against the polishing pad,distributes a downward pressure across the back surface of the wafer,transfers torque from the drive shaft to the wafer, and ensures that thewafer does not slip out from beneath the carrier head during polishingoperations. A description of a carrier head can be found in U.S. PatentPublication No. 2006-0154580, the entire disclosure of which isincorporated herein by reference. In addition, the carrier head 80 canbe configured to laterally oscillate across the polishing pad, e.g.,move along a radius of the polishing pad.

A polishing liquid, e.g., a slurry 38 containing abrasive particles, canbe supplied to the surface of polishing pad 30 by a slurry supply portor combined slurry/rinse arm 39.

In typical operation, the platen is rotated about its central axis 25,and the carrier head 80 is rotated about its central axis 81 andtranslated laterally across the surface of the polishing pad.

The polishing apparatus 20 also includes an in-situ optical monitoringsystem 40, which can be used to determine a polishing endpoint of thewafer being polished, as will be discussed below. The optical monitoringsystem includes a light source 44 and a light detector 46. Light passesfrom the light source 44, through an optical access 36 in the polishingpad 30, impinges and is reflected from the substrate 10 back through theoptical access 36, and travels to the light detector 46.

The optical access 36 through the polishing pad 30 to the substrate canbe provided by an aperture in the pad or a solid window. The solidwindow can be secured to the polishing pad, although in someimplementations the solid window can be supported on the platen 24 andproject into an aperture in the polishing pad. If the optical access 36is in the form of a solid window, the solid window can include, forexample, a rigid crystalline or glassy material (e.g., quartz or glass),a softer plastic material (e.g., silicone, polyurethane or a halogenatedpolymer such as a fluoropolymer), or a combination of these materials.The solid window can be transparent to white light or light(s) at otherwavelengths.

A bifurcated optical cable 54 can be used to transmit the light from thelight source 44 to the optical access 36 and back from the opticalaccess 36 to the light detector 46. The bifurcated optical cable 54 caninclude a “trunk” 55 and two “branches” 56 and 58.

The in-situ optical monitoring system 40 can include an optical assembly53 that is removably secured to the platen 24 in a recess 26 in theplaten 24 so that the optical assembly 53 rotates with the platen 24.The optical access 36 can be aligned with the recess 26 and the opticalassembly 53. The recess 26 and the optical access 36 can be positionedsuch that they have a view of the substrate 10 during a portion of theplaten's rotation, regardless of the translational position of thecarrier head. The optical assembly 53 can hold one end of the trunk 55of the bifurcated fiber optic cable 54, which is configured to conveylight to and from a substrate surface being polished. The optical head53 can include one or more lenses to focus or collimate the light beam.The optical head 53 can also include a window overlying the end of thebifurcated fiber optic cable 54. Alternatively, the optical assembly 53can merely hold the end of the trunk 55 adjacent the solid window in thepolishing pad. A refractive index gel can be applied to a bottom surfaceof the window so as to provide a medium for light to travel from thetruck of the fiber optic cable to the window.

The in-situ optical monitoring system 40 can also include an in-situmonitoring module 50 that is removably secured to the platen 24. Thein-situ monitoring module 50 can include one or more of the following:the light source 44, the light detector 46, and circuitry for sendingand receiving signals to and from the light source 44 and light detector46. For example, the output of the detector 46 can be a digitalelectronic signal that passes through a rotary coupler, e.g., a slipring, in the drive shaft 22 to the controller for the optical monitoringsystem. Similarly, the light source can be turned on or off in responseto control commands in digital electronic signals that pass from thecontroller through the rotary coupler to the module 50.

The in-situ monitoring module can also hold the respective ends of thebranch portions 56 and 58 of the bifurcated optical fiber 54. The lightsource 44 is operable to transmit light, which is conveyed through thebranch 56 and out the end of the trunk 55 located in the optical head53, and which impinges on a substrate being polished. Light reflectedfrom the substrate is received at the end of the trunk 55 located in theoptical head 53 and conveyed through the branch 58 to the light detector46.

In one implementation, the bifurcated fiber cable 54 is a bundle ofoptical fibers. The bundle includes a first group of optical fibers anda second group of optical fibers. An optical fiber in the first group isconnected to convey light from the light source 44 to a substratesurface being polished. An optical fiber in the second group isconnected to received light reflecting from the substrate surface beingpolished and convey the received light to a light detector. The opticalfibers can be arranged so that the optical fibers in the second groupform an X-like shape that is centered on the longitudinal axis of thebifurcated optical fiber 54 (as viewed in a cross section of thebifurcated fiber cable 54). Alternatively, other arrangements can beimplemented. For example, the optical fibers in the second group canform V-like shapes that are mirror images of each other. A suitablebifurcated optical fiber is available from Verity Instruments, Inc. ofCarrollton, Tex.

There is usually an optimal distance between the polishing pad windowand the end of the trunk 55 of bifurcated fiber cable 54 proximate tothe polishing pad window. The distance can be empirically determined andis affected by, for example, the reflectivity of the window, the shapeof the light beam emitted from the bifurcated fiber cable, and thedistance to the substrate being monitored. In one implementation, thebifurcated fiber cable is situated so that the end proximate to thewindow is as close as possible to the bottom of the window withoutactually touching the window. With this implementation, the polishingapparatus 20 can include a mechanism, e.g., as part of the opticalassembly 53, that is operable to adjust the distance between the end ofthe bifurcated fiber cable 54 and the bottom surface of the polishingpad window. Alternatively, the proximate end of the bifurcated fibercable is embedded in the window.

The light source 44 is operable to emit a broad wavelength band oflight, e.g., white light. In some implementations, the white lightemitted includes light having wavelengths of 200-800 nanometers. Asuitable light source is a xenon lamp or a xenon-mercury lamp. In someimplementations, the light source generates infrared or ultravioletlight.

The light detector 46 can be a spectrometer. A spectrometer is basicallyan optical instrument for measuring properties of light, for example,intensity, over a portion of the electromagnetic spectrum. A suitablespectrometer is a grating spectrometer. Typical output for aspectrometer is the intensity of the light as a function of wavelength.

Optionally, the in-situ monitoring module 50 and optical assembly 53 caninclude additional other sensor elements in addition to thespectrometer, such as an eddy current sensor, a monochromaticinterferometric optical sensor, or a friction sensor.

The light source 44 and light detector 46 are connected to a computingdevice 48 operable to control their operation and to receive theirsignals. The computing device can include a microprocessor situated nearthe polishing apparatus, e.g., a programmable computer, such as apersonal computer. The computing device can, for example, synchronizeactivation of the light source 44 with the rotation of the platen 24.

As shown in FIG. 2, the optical monitoring system can make a sequence ofspectral measurements as the optical assembly 53 and optical access 36scan across the substrate. Each of points 201-211 represent a locationon the substrate 10 where light from the in-situ monitoring systemimpinges and reflects off to provide a spectral measurement. As shown inFIG. 2, the locations can trace an arc across the substrate due to therotation of the platen 24. Optionally, the computer can cause the lightsource 44 to emit a series of flashes starting just before and endingjust after the substrate 10 passes over the optical access 36 module,with each flash corresponding to a measurement location. Alternatively,the computer can cause the light source 44 to emit light continuouslystarting just before and ending just after the substrate 10 passes overthe in-situ monitoring module.

The computing device 48 can be programmed to store spectral intensitymeasurements from the detector, to display the spectra on an outputdevice, to calculate the remaining thickness, amount removed, andpolishing rate from the spectral intensity measurements, and/or todetect the polishing endpoint. The computing device 48 also can beconfigured to cause, for example, the polishing rate and polishing timeof the polishing apparatus to be adjusted based upon the received light.

Generally, in order to calculate a thickness of a layer on the substrateor to detect a polishing endpoint based on the spectrum measured by theoptical monitoring system 40, a measured spectrum is compared to alibrary of reference spectra.

FIG. 3 shows an exemplary process 300 for generating a library thatassociates reference spectra with substrate characteristics.

Initially, at least one characteristic of a reference substrate, e.g.,of a substrate layer, is measured at multiple locations on the referencesubstrate (step 302). For each location, the measured characteristic andthe location of the measurement are stored, e.g., in a first datastructure in a computer-readable medium.

The reference substrate should have the same pattern and die featuregeometry as an actual product substrate would have at the same point inthe manufacturing process, although the reference substrate need notitself be intended to be a product substrate. The characteristic shouldbe measured for at least a substrate that has approximately thethickness as the product substrate will have when measured by aspectrographic system that will use the library. For example, if theproduct substrate will be measured by an in-line system pre orpost-polishing, then the reference substrate should be measured withapproximately the expected pre or post-polishing thickness,respectively. If the product substrate will be measured by an in-situmonitoring system, then the reference substrate should be measured forat least the desired post-polishing substrate layer thickness, but asdiscussed below, the characteristic can be measured for one or morereference substrates at multiple different stages of polishing of thesubstrate layer.

The characteristic can be a physical property of the substrate thatimpacts the performance of circuitry on the substrate. An exemplaryphysical characteristic is a thickness of a film of interest, e.g., theoutermost layer undergoing processing. Other thickness-derivedcharacteristics can include step height or erosion. Other possiblephysical characteristics of the film include conductivity.Alternatively, the characteristic can be a manufacturing metric, e.g., ayield. In addition, the film of interest need not be the outermostlayer, e.g., the physical characteristic can be a thickness of anunderlying layer.

The substrate characteristics can be measured using a metrology systemthat provides precise positioning of a sensor to a desired measurementlocation on the substrate. The metrology system can be part of anin-line or stand-alone metrology station. The metrology station caninclude positional sensors and alignment mechanism for aligning thesubstrate and the sensors so that the same location is repeatedly andaccurately measured for different substrates. If the metrology systemmeasures substrate layer thickness, it can be a non-contact opticalmetrology system, such an optical metrology system that uses spectralintensity and/or polarization information to calculate layer thickness,or it can be a contact profilometer. If the metrology system measuressubstrate layer conductivity, it can include a four-point probe.Suitable optical metrology systems for measuring the substrate layerthickness are available from Nova Measuring Instruments and Nanometrics.

The characteristic is measured at a multiple locations of interest onthe reference substrate. In some implementations, these locations are“well-defined” points, i.e., locations at which a metrology device cangenerate an accurate and reliable measurement without relying on thisinvention. For example, in the context of a conventional non-contactoptical metrology device, a well defined location is a location at whichthe optical model used by the metrology device can be used to accuratelycalculate the substrate layer thickness a priori from the measuredproperties of the reflected light (e.g., spectral intensity andpolarization) with a reasonable amount of computational processingpower. For example, in the context of a conventional four-point probe, awell defined location is a location with sufficiently large conductivearea for placement of the probe. Locations having a lower density ofgeometrical features than other discrete regions of the wafer can beselected as well-defined points. For example, well-defined points mayinclude regions in which bond pads are placed, or regions in whichsurfaces of uniform material composition are formed.

The well-defined points can be selected so that each measurement on aparticular substrate occurs for locations in different dies but at thesame relative position within each die. For a particular substrate at aparticular stage of polishing, the number of locations measured can beequal to or less than, e.g., less than, the number of dies on thesubstrate. The measurement locations can be selected to be generallyuniformly spaced across the substrate.

FIG. 4 illustrates a reference wafer having exemplary well-definedpoints. Referring to FIG. 4, the reference wafer 406 may contain one ormore die features 402 (exemplary dies are labeled D₁, D₂ . . . D_(m-1)and D_(m)). To provide accurate thickness profile analysis of thereference wafer 406, a thickness from each well-defined point 402(exemplary well-defined points are labeled WP₁, WP₂ . . . WP_(m-1) andWP_(m)) is measured. Specifically, light is impinged upon eachwell-defined point, as shown by the measurement spot 404, and portionsof the light reflected off the well-defined points 300 a-300 f arereceived. Based on spectra detected in the reflected light, thicknessmeasurement at these well-defined points 300 a-300 f can be obtained.

FIG. 5 illustrates a first data structure generated from collected datathat associates the coordinates of at least some of, and possibly each,well-defined point with a corresponding substrate characteristic.Referring to FIGS. 4 and 5, the substrate characteristic, such asthicknesses, of the reference wafer is measured at well-defined pointsWP₁, WP₂ . . . WP_(m-1) and WP_(m) positioned at coordinates (Xw₁, Yw₁),(Xw₂, Yw₂) . . . (Xw_(m-1), Yw_(m-1)) and (Xw_(m), Yw_(m)),respectively. Of course, a different coordinate system (e.g., R, θ)could be used.

As shown, wafer characteristic T₁ is measured for well-defined point WP₁at coordinates (Xw₁, Yw₁). Similarly, wafer characteristics T₂, . . .T_(m-1) and T_(m) are measured for well-defined points WP₂ . . .WP_(m-1) and WP_(m) at coordinates (Xw₂, Yw₂), . . . (Xw_(m-1),Yw_(m-1)) and (Xw_(m), Yw_(m)), respectively. These measurements canthen be stored in the first data structure. If the substratecharacteristics were calculated from measured spectra, then the datastructure can optionally also store the measured spectrum associatedwith each coordinate. In addition, for each measurement or group ofmeasurements, the data structure can store a unique identifier of thereference substrate, and data indicating the stage of polishing of thereference substrate layer (e.g., an elapsed polishing time or a numberof platen rotations).

In some implementations, substrate characteristic are calculated for atleast some intermediate points. These intermediate points can have thesame relative positioning within each die as the well-defined points.The intermediate points can be well-defined points at which thesubstrate characteristic was not measured, but can also be other pointsin a die.

The substrate characteristic of the intermediate points can becalculated by linear interpolation or extrapolation from measuredwell-defined points, particularly the nearest several measuredwell-defined points, e.g., nearest two to four well-defined points, onthe reference substrate. For example, referring to FIG. 4, if substratelayer thicknesses T₁ and T_(m-1) are measured for points WP₁ andWP_(m-1), and the well-defined and intermediate points are uniformlyspaced, then the thickness for intermediate point IP can be calculatedas the average of T₁ and T_(m-1). More generally, the linearinterpolation can be a weighted average of nearby measured well-definedpoints with weighting based on relative distance to the well-definedpoints.

Referring back to FIG. 3, at step 304, spectra are measured at multiplelocations across the reference substrate. For each location, the spectraand the location of the measurement are stored, e.g., in a datastructure in a computer-readable medium.

The spectra are measured for at least some locations (hereinafter“measurement points”) other than the well-defined points, although it ispermissible for spectra to also be measured at locations that overlapwith the well-defined points. However, the measurement points need notselected so that each measurement occurs at the same relative positionwithin a die.

The spectra can be measured with an optical monitoring system that doesnot provide precise positioning of a sensor to a desired measurementlocation on the substrate. For example, the spectra can be measured withan optical monitoring system that scans a sensor across the substrate atrelatively high speed (e.g., across a 300 mm diameter wafer in less than10 seconds, e.g, in less than 5 seconds), and without halting. Theoptical monitoring system can be part of an in-situ monitoring system,e.g., at a polishing station, or an in-line metrology station. Thespectra can measured using an optical monitoring system withsubstantially the same configuration as the in-situ monitoring system tobe used at the polishing system (e.g., as described above with referenceto FIG. 1). In one implementation, the spectra are measured using thesame in-situ optical monitoring system as the one that will be used inthe polishing system. In another implementation, the monitoring systemcan be an in-line or stand alone system that otherwise mimics thein-situ monitoring system, e.g., using the same light source, detector,sampling rate, fiber optic connector and window, but is not in apolishing station.

For a particular substrate at a particular stage of polishing, thenumber of measurement points can be greater than the number of measuredwell-defined locations, and can be much greater, e.g., ten or more timesgreater, e.g., one-hundred or more times greater. At least some diesinclude more than one measurement point. In general, the spacing betweenmeasurement points is less than the spacing between the well-definedlocations, and the density of measurement points is also greater thanthe density of the well-defined locations. The number of measurementpoints can be greater than the number of dies on the substrate.

For example, referring to FIG. 6, assuming that the spectra are measuredusing an in-situ optical monitoring system as described with referenceto FIG. 1 above, the light beam creates a sweeping path 610 and spectraare measured along the sweeping path, as indicated by the measurementpoints 612 MP₁, MP₂ . . . MP_(m-1) and MP_(m).

The number of measurement points can depend on the sampling rate of thedetector 46. The detector 46 can have a sampling rate between about 10and 100 Hz, corresponding to a sampling period between about 2.5 and 100milliseconds. Each time the detector 46 is sample, the in-situ opticalmonitoring system 40 retrieves spectral data, such as intensity andreflectance data, from an associated measurement point 612. Thecomputing device 48 can cause the light source 44 to emit a series oflight beam starting just before and ending just after the referencewafer 406 passes over the optical module 53, or the light beam can be oncontinuously.

Although FIG. 6 shows only eleven measurement points MP₁, MP₂, . . .MP₁₀ and MP₁₁, this is illustrative and there could be many moremeasurement points. The number of measurement points depends on theplaten rotation rate and the sampling rate of the detector 46. Ofcourse, a lower triggering rate can result in fewer (and more widelyspaced) measurement points, whereas a faster triggering rate can resultin a larger number of (and more closely spaced) measurement points.Similarly, a lower rotation rate can result in a larger number ofmeasurement points, whereas a faster rotation rate can result in fewermeasurement points.

Also, more than a single sweep can be performed on a particularreference substrate at a particular stage of polishing to produce ameasurement points. From the measurement points, the computing device 48accumulates a set of intensity or reflectance measurements, eachassociated with a measurement time (e.g., time between a previous sweepand a subsequent sweep).

Spectra from the measurement points 612 can be collected using anoptical monitoring tool capable of producing measurement in broadwavelength range, covering, for example, the deep ultraviolet (e.g.,wavelengths below 300 nm), ultraviolet, visible or infrared wavelengthregions. The wavelength range in which measurement is to be taken caninclude an entire or a partial segment of the in-situ optical monitoringsystem's operating wavelength range.

For illustrative purposes, spectra S₁, S₂, . . . S₈, S₉ . . . S_(m-1)and S_(m) are measured at measurement points MP₁, MP₂ . . . , MP₈, MP₉ .. . MP_(m-1) and MP_(m) positioned at coordinates (Xm₁, Ym₁), (Xm₂, Ym₂). . . (Xm₈, Ym₈), (Xm₉, Ym₉) . . . (Xm_(m-1), Ym_(m-1)) and (Xm_(m),Ym_(m)), respectively.

FIG. 7 illustrates a second data structure generated from collected datathat associates the coordinates of each measurement point with acorresponding spectrum. As shown, spectra S₁ is measured at coordinates(Xm₁, Ym₁). Similarly, spectra S₂ . . . S_(m-1) and S_(m) are measuredat coordinates (Xm₂, Ym₂) . . . (Xm_(n-1), Ym_(n-1)) and (Xm_(n),Ym_(n)) respectively. Of course, a different coordinate system (e.g., R,θ) could be used.

The coordinate position of each measurement point at which a spectrum isobtained can be determined by using methods similar to those describedin U.S. Pat. Nos. 7,018,271, 7,097,537, and 7,153,185 the disclosures ofwhich is incorporated herein by reference. In particular, thesedisclosures describe calculation of a radial positions of a measurement,and an angular position can be calculated from a carrier head angularposition at the time of measurement, e.g., as sensed by a rotaryencoder. Of course, the R, θ coordinate determination can be transformedinto another coordinate system (e.g., X, Y).

In addition, for each measurement or group of measurements, the seconddata structure can store a unique identifier of the reference substrate,and data indicating the stage of polishing of the reference substratelayer (e.g., an elapsed polishing time or a number of platen rotations).

Returning to FIG. 2, at step 206, spectra measured from measurementpoints are associated with substrate characteristics based onpredetermined conditions. The associated spectra and substratecharacteristics are stored to form a library. For example, each spectrumcan be linked to a substrate characteristic of a nearby well-definedpoint based on the coordinates of the measurement point at which thespectrum was measured. Associating spectra with substratecharacteristics will be described in further detail below with referenceto FIG. 9.

FIG. 9 illustrates an exemplary process 900 for associating spectra withwafer characteristics. A set of well-defined points can be determinedfor use in generating the library (step 902). Typically, for aparticular substrate at a particular stage of polishing, all of thewell-defined points at which the substrate characteristic was measuredwould be used, but it is possible for fewer than all of the well-definedpoints to be used to generate the library. Similarly, a set ofmeasurement points is determined for use in generating the library (step904). Again, typically for a particular substrate at a particular stageof polishing, all of the measurement points at which spectra weremeasured would be used, but it is possible for fewer than all of themeasurement points to be used to generate the library.

For each measurement point in the set, one of well-defined points isselected, and the substrate characteristic of the selected well-definedpoint is assigned to the spectra of the measurement point in the library(step 904). The selected well-defined point is near the measurementpoint, e.g., one of the four closest measurement points. In oneimplementation, the well-defined point closest to the measurement pointis selected. This can be accomplished by comparing the coordinates ofthe measurement point to the coordinates of well-defined points and/orcalculating distances between the measurement point and the well-definedpoints. Once the distance between a measurement point and neighboringwell-defined points are determined, an association can be established byidentifying a well-defined point closest to the measurement point, andlinking the spectrum previously measured at that well-defined point tothe wafer characteristic(s) associated with the measurement point. Inanother implementation, the selected well-defined point is thewell-defined point in the same die as the measurement point.

As an example, referring to FIG. 6, assuming that coordinates (Xm₁, Ym₁)and (Xm₂, Ym₂) of measurement points MP₁ and MP₂ are closest towell-defined points WP₁ and coordinates (Xm₈, Ym₈) and (Xm₉, Ym₉) ofmeasurement points MP₈ and MP₉ are closest to well-defined points WP₂,then associations between spectra S₁ and S₂ and wafer characteristic T₁,and between spectra S₈ and S₉ and wafer characteristic T₂ areestablished (see FIG. 8). Of course, associates between the spectra forthe other measurement points and substrate characteristics for otherwell-defined points can also be made.

In some implementations, to expedite the process of distancedetermination, a predetermined distance or zone from a well-definedpoint can be identified in advance so that spectra measured atmeasurement points falling within the predetermined distance or zone areautomatically recognized and associated with the wafer characteristicsat that well-defined point. For example, still referring to FIG. 6, aspectrum of any measurement point falling inside a first zone 602 a isautomatically associated with the wafer characteristics of thewell-defined point WP₁, and spectrum of any measurement point fallinginside a second zone 602 b is automatically associated with the wafercharacteristics of the well-defined point WP₂. The definition of thezone for each well-defined point can be stored in the first datastructure.

In these implementations, associations for spectra of measurement pointsfalling inside an overlapping region of both the first and second zonescan be established by using the distance technique discussed above. Forexample, measurement point MP₇ is situated between the boundaries of thefirst zone 602 a and the second zone 602 b. If the distance between themeasurement point MP₇ and the well-defined point WP₁ is shorter thanthat between the measurement points MP₇ and the well-defined point WP₂,then the association between the spectrum at the measurement point MP₇and substrate characteristics at the well-defined point WP₁ isestablished. Conversely, if the distance between the measurement pointMP₇ and the well-defined point WP₁ is longer than that between themeasurement point MP₇ and the well-defined point WP₂, then theassociation between the spectrum at measurement point MP₇ and substratecharacteristics at the well-defined point WP₂ is established.

FIG. 8 illustrates a third data structure generated from collected datathat associates spectra with substrate characteristics and that formsthe library. As shown, spectrum S₁ is associated with thickness T₁,spectrum S₂ is associated with thickness T₁, spectrum S₈ is associatedwith thickness T₂ and spectrum S₉ is associated with thickness T₂.Optionally, information related to the distance between each measurementpoint and well-defined point, including coordinates thereof, can bestored in the library.

Returning to FIG. 2, at step 208, it is determined whether spectra andsubstrate characteristic measurements of a reference substrate areneeded at additional different polishing stages. If it is determinedthat measurements are needed at additional different polishing stages(“Yes” branch of step 208), steps 202-206 are repeated. In general,steps 202-206 can be repeated until spectra and substratecharacteristics are accumulated for a sufficient number of differentthicknesses to ensure reliable operation during polishing of actualproduct wafers.

In one implementation, the reference substrate is initially measured ata partially polished state. After substrate characteristics and spectrahave been measured in, the reference substrate can be transferred backto the polishing apparatus to partially polish an additional incrementalamount of substrate layer material. In fact, spectra can be collectedduring the polishing process (e.g., using the in-situ monitoring systemdescribed above to collect spectra from the last platen rotation beforepolishing is halted). The reference substrate is then removed from thepolishing apparatus for measurement of the substrate characteristics atthe well-defined locations, e.g., using a conventional in-line orstand-alone metrology system. Of course, the reference substrate canthen be sent back to the polishing system for additional polishing.

Otherwise (at “No” branch of step 208), process 200 indicates that thelibrary is prepared to be used for processing actual product wafers(step 210).

Steps 202 and 204 can be performed in the order listed or in reverse ofthe order listed. Thus, spectra measurement at multiple measurementpoints across the reference substrate can be performed before or afterthe measurement of substrate characteristics at well-defined points. Inaddition, in some implementations, some operations of steps 202-206 canbe performed in another order or in parallel to achieve the same result.For example, an association between spectrum measurements and wafercharacteristics can be performed as each spectrum is received. Asanother example, if the substrate characteristics are calculated forsome of the well-defined points (e.g., by linear interpolation), thecalculation can be performed after the closest well-defined point hasbeen identified for a spectrum.

The library can reside in the memory of the computing device 48. Thelibrary can be updated with new data (e.g., if a product substrate isdirected to a metrology station, then spectra from the product substratecollected from the in-situ monitoring system, e.g., from the last platenrotation before polishing was halted, could be associated with thesubstrate characteristics measured at the metrology station). Ifdesired, the library also can include spectra that are not collected butare theoretically generated. Other parameters such as time in which thespectra are measured also can be stored in the library. In addition, thelibrary is not limited to storing data collected from a singlesubstrate, and can include spectra collected from multiple substrates.

Because precise alignment of the measurement tools at the well-definedpoints is no longer required, the library can significantly increase theoverall speed with which substrate characteristics can be determined,and thus the throughput of the polishing apparatus can be increased. Tooptimize the throughput of the polishing apparatus, a high density ofspectra and wafer characteristics covering an entire wafer area arecaptured before, during and after polishing so that a sufficient numberof wafer characteristics and spectra measurements is stored. Thisenables high speed, high volume, precise real time thickness extractionand reporting. However, if during polishing of an actual product wafer,a measured spectrum is found not to have a matching spectrum stored inthe library, the library can be immediately updated to include themeasured spectrum and its associated wafer information.

Once a sufficient number of established associations are identified andcollected, the library can be used for monitoring during processing ofactual product wafers. During actual processing, the optical monitoringsystem sweeps across a product substrate and measures a sequence ofspectra from the reflected light, and the library can be searched for amatching spectra. The search may include direct comparison of themeasured spectra to those stored in the library, or using a combinationof searching and fitting algorithms. The substrate characteristicsassociated with the spectra selected from the search can then be usedfor monitoring or control of the polishing process.

In some implementations, endpoint can be called when a measured spectrumhas a desired substrate characteristic. For example, as discussed above,for the spectra measured during polishing, the closest matching spectrumin the library can be identified, e.g., using searching and/or fittingalgorithms. If the substrate characteristics, e.g., thickness, of thematching spectrum in the library has the desired characteristic, e.g., adesired thickness, then the polishing endpoint is triggered.

In another implementation, the library is searched in advanced for adesired endpoint criterion, e.g., a desired thickness, and one or morespectra which have a substrate characteristic with the desired criterionare identified as desired spectra. Then, during polishing, for thespectra measured during polishing, the closest matching spectrum in thelibrary can be identified. Polishing can be halted when the measuredspectrum matches a desired spectra from the library.

In some implementations, the library is not used for endpointdetermination, but is merely used for monitoring and/or feedback controlof pressure applied by the carrier head to the substrate. For example,endpoint could be detected using a difference traces between the currentspectra measured during polishing and a reference spectrum, as describedin U.S. Patent Application Publication No. 2007/0042675, the disclosureof which is incorporated herein by reference in its entirety.

Optionally, the spectra collected can be verified to enhance thereliability of the library. FIG. 10 illustrates an exemplaryverification process 1000 for the library. Referring to FIG. 10, thelibrary is sorted (step 1000). This operation functions to expedite theverification process of the data stored in the library. If desired, thisoperation can be bypassed if the library contains less than apredetermined number of data. Once the library is sorted, spectra(and/or other wafer parameters) stored in the library are analyzed (step1004). If it is determined that two or more spectra stored in thelibrary are substantially identical yet exhibit a different thickness(step 1006), then both spectra are permanently discarded from thelibrary (“Yes” branch of step 1008). Otherwise (“No” branch of step1008), the analysis step is resumed.

In some implementations, spectra stored in the library are normalized,averaged and/or filtered to enhance the reliability of the library. Forexample, spectra matching can be performed after processing andfiltering the measured spectra (e.g., using high pass filter or low passfilter) to remove noise and interference. The spectra also can becompensated for optical system distortions and other artifacts, or bematched to different optical response used to collect the spectra forthe library. This may include, for example, intensity variations andwavelength dependent scattering due to the feature structure, arraydimensions, numerical aperture effects, wavelength range andpolarization.

In some implementations, each measured raw spectra can be normalized toremove light reflections contributed by mediums other than the film orfilms of interest. Normalization of spectra facilitates the comparisonprocess discussed above. Light reflections contributed by media otherthan the film or films of interest include light reflections from, forexample, the polishing transparent window 36 and from the base siliconlayer of the wafer. Contributions from, for example, a transparentwindow 36 can be estimated by measuring the spectrum of light receivedby the in situ optical monitoring system 40 under a dark condition(i.e., when no wafers are placed over the in situ optical monitoringsystem 40). Contributions from, for example, the silicon layer can beestimated by measuring the spectrum of light reflecting off a baresilicon wafer. The contributions can be obtained prior to commencementof the polishing step.

A measured raw spectrum can be normalized as follows:

normalized spectrum=(A−Dark)/(Si−Dark)

where A is the raw spectrum, Dark is the spectrum obtained under thedark condition, and Si is the spectrum obtained from the bare siliconwafer.

Optionally, the collected spectra can be sorted based on the region ofthe pattern that has generated the spectrum, and spectra from someregions can be excluded from the endpoint calculation. In particular,spectra that are from light reflecting off scribe lines can be removedfrom consideration. Different regions of a reference wafer usually yielddifferent spectra (even when the spectra were obtained at a same pointof time during polishing).

For example, a spectrum of the light reflecting off a scribe line in awafer can be different from the spectrum of the light reflecting off anarray of the wafer. Because of their different shapes, use of spectrafrom both regions of the pattern usually introduces error into theendpoint determination. However, the spectra can be sorted based ontheir shapes into a group for scribe lines and a group for arrays.Because there is often greater variation in the spectra for scribelines, usually these spectra can be excluded from consideration toenhance precision.

A high pass filter also can be applied to the measured raw spectra.Application of the high pass filter can remove low frequency distortionof the average of the subset of spectra. The high pass filter can beapplied to the raw spectra, their average, or to both the raw spectraand their average.

In some implementations, based on the current spectra of each zone andthe variations thereof, the computing device 48 can determine theflatness of the wafer and the polishing uniformity for CMP tool andprocess qualification. For example, the computing device 48 can appliesprocess control and endpoint detection logic to determine when to changeprocess and polish parameter and to detect the polishing endpoint.Possible process control and endpoint criteria for the detector logicinclude local minima or maxima, changes in slope, threshold values inamplitude or slope, or combinations thereof. The spectra of lightreflected from a wafer can be frequently monitored and collected aspolishing progresses. Based on the reflected spectra, the computingdevice 48 can determine an endpoint of a polishing process.

If more than one current spectra is measured for a platen revolution,then the spectra can be grouped, combined, e.g., averaged within eachgroup, and the averages are designated to be current spectra. Thespectra can be grouped by radial distance from the center of the wafer.By way of example, for a given platen rotation, a first current spectrumcan be obtained, e.g., by averaging, from spectra measured as points 211and 219 (FIG. 3), a second current spectrum can be obtained from spectrameasured at points 212 and 218, a third current spectra can be obtainedfrom spectra measured at points 213 and 217, and so forth.

FIG. 11 shows another method 1100 for determining an endpoint of apolishing step. Initially, index values are assigned to the spectra inthe library (step 1104). The index values can be selected tomonotonically increase as polishing progresses, e.g., an index valuescan be proportional to a number of platen rotations. Thus, each indexnumber can be a whole number, and the index number can represent theexpected platen rotation at which the associated spectrum would appear.The library can be implemented in memory of the computing device of thepolishing apparatus.

A wafer from the batch of wafers is polished, and the following stepsare performed for each platen revolution. One or more spectra aremeasured to obtain a current spectra for a current platen revolution(step 1106). The spectra are obtained as described above. The spectrastored in the library which best fits the current spectra is determined(step 1108). The index of the library spectrum determined to best fitsthe current spectra is appended to an endpoint index trace (step 1110).Endpoint is called when the endpoint trace reaches a reference index,e.g., the index of a spectrum having the desired thickness or othersubstrate characteristic (step 1112).

Although implementations for determining a film thickness have beendescribed, other parameters including shallow trench depth, step heightof various semiconductor materials (e.g., silicon dioxide, siliconnitride), an area of trench or active region of the wafer, or thicknessof silicon dioxide or pad layers.

Although the discussion above focuses on use of the library in apolishing endpoint detection system, the library could also be used infor an in-line spectrographic metrology system, e.g., an in-line systemthat scans a sensor across the substrate at relatively high speed. Thisin-line metrology system could be used before or after processing, e.g.,polishing, of the substrate, and the substrate characteristics derivedfrom the measured could be used for feed-forward or feed-back control ofthe polishing system. For example, if the library associates thicknesseswith spectra, then the in-line metrology system could measure substratelayer thickness at multiple points along a radius or diameter of thesubstrate prior to polishing, and the measured layer thickness datacould be used to control the polishing system (e.g., select endpointcriteria or polishing head pressures) during polishing of thatsubstrate. As another example, the in-line metrology system couldmeasure substrate layer thickness at multiple points along a radius ordiameter of the substrate after polishing, and the measured layerthickness data could be used to control the polishing system (e.g.,select endpoint criteria or polishing head pressures) during polishingof a subsequent substrate. Due to the large number of spectra stored inthe system, the system can provide reliable measurements of thesubstrate characteristics without precise positioning of the sensor toany well-defined point, thereby permitting the measurements to be madeat the in-line station at high throughput.

An implementation of a substrate processing system 8 that includes anin-line spectrographic metrology system 500 is illustrated in FIG. 12.The substrate processing system 8 includes the chemical mechanicalpolishing apparatus 20, a factory interface module 100, a wet robot 140,and a cleaner 170. Substrates 10, e.g., silicon wafers with one or morelayers deposited thereon, are transported to the substrate processingsystem 8 in cassettes 12, and are extracted from the cassettes 12 by thefactory interface module 100 for transport to the polishing apparatus 20and the cleaner 170. The operations of the substrate processing system 8are coordinated by controller 48, such as one or more programmabledigital computers executing control software. Some of the modules, suchas the wet robot 140 and cleaner 170, could be omitted, depending on theconfiguration of the processing system, and the processing system couldinclude other modules, such as a deposition or etching apparatus.

The polishing apparatus 20 can includes a series of polishing stations150 and a transfer station 152. The transfer station 152 serves multiplefunctions, including receiving individual substrates 10 from the wetrobot 140, washing the substrates and loading the substrates intocarrier heads. Each polishing station can includes a rotatable platenholding a polishing pad 30. Different polishing pads can be used atdifferent polishing stations. A rotatable carousel 154 that holds fourcarrier heads 80 is supported above the polishing stations (drivesystems above the carrier heads and the carrier head over the transferstation are not illustrated in FIG. 12 to provide a clearer top view).The carousel 154 rotates to carry the substrates between the polishingstations 150 and the transfer station 152.

The cleaner 170 can be generally rectangular shaped cabinet with a frontwall 171, a back wall 172, and two side walls 174. The interior of thecleaner 170 is divided into an input or staging section 176 and acleaning section 178. The staging section 176 includes a substrate-passthrough support 180 and an indexable buffer 182, each of which can holdone or more substrates in a vertical orientation. The cleaner alsoincludes a walking beam 184 which can hold a substrate in a verticalorientation.

The wet robot 140 is configured to transport the substrate between thestaging section 176 and the polishing apparatus 20.

The factory interface module 100 can be substantially rectangular inshape and include an outer wall 101, an inner wall 102, a first sidewall 104, and a second side wall 106. The outer wall 101 can be alignedwith a cleanroom wall. A plurality (e.g., four) cassette support plates110 project from the outer wall 101 into the cleanroom to accept thecassettes 12, and a plurality of cassette ports 112 are formed in theouter wall 101 to permit transport of the substrates from the cassettes12 into the factory interface module 100. The inner wall 104 matesagainst a front wall 171 of the cleaner 170 and shares an entry port 120(to the staging section 176) and an exit port 122 (from the end of thecleaning section 178) with the cleaner front wall 171. The inner wall102 and the cleaner front wall 170 may be combined into one structure,and there may be additional ports from the factory interface module 100to the cleaner 170.

One or more factory interface wafer handlers 130 (hereinafter simply“robot”), depicted in greater detail in FIGS. 13 and 14, are housedwithin the factory interface module 100. In some implementations thefactory interface robot 130 has a base 132, a rotatable vertical shaft134 extending from the base 132, a horizontally extendible articulatedarm 136 supported by the shaft 134, a rotary actuator 138 at the end ofthe articulated arm 136, and a substrate gripper 139 (in phantom belowthe substrate 10 in FIG. 13) supported by the rotary actuator 138. Thevertical shaft 134 is capable of lifting and lowering the articulatedarm 136 vertically. Rotation of the vertical shaft 134 permits rotarymotion of the articulated arm 136 about a vertical axis, and thearticulated arm 136 is configured to extend and retract horizontally.The rotary actuator can be pivotally connected to the end of thearticulated arm 136 so as to be rotatable about a vertical axis. Inaddition, the rotary actuator 138 can rotate the substrate gripper 139about a horizontal axis. The factory interface robot 130 thus provides awide range of motion to manipulate the substrate held by the gripper139. The gripper 139 can be a vacuum chuck, an electrostatic chuck, anedge clamp, or similar wafer gripping mechanism. The factory interfacerobot can also include an optical detector to sense whether a substrateis being held by the gripper 140. Sensors, e.g., encoders, can be usedto detect the position of the movable elements of the robot 130 so thatthe position of the gripper 139 and substrate 10 can be calculated.

The base 132 can be supported on a linear rail 131 that extends parallelto the inner and outer walls 102, 100. A motor can drive the factoryinterface robot 130 laterally along the rail 131 to access the entryport 120, the exit port 122, the cassette ports 112 (FIG. 12 illustratestwo positions along the slide 142 for the factory interface robot 130),and the in-line spectrographic metrology system 500 within the factoryinterface 100.

As shown in FIG. 14, the in-line spectrographic monitoring system 500operates similarly to the in-situ optical monitoring system, andincludes a light source 44 and a light detector 46. Light passes fromthe light source 44, through an optical guide, impinges and is reflectedfrom a substrate 10 held in the factory interface 100, back through theoptical guide, and travels to the light detector 46. As with the in-situsystem, a bifurcated optical cable 54 can be used to transmit the lightfrom the light source 44 to the substrate 10 and back from the substrate10 to the light detector 46. The bifurcated optical cable 54 can includea “trunk” 55 with an end 504 fixed in a position selected to be inproximity to substrate when the substrate is to be scanned by themetrology system, and two “branches” 56 and 58 connected to the lightsource 44 and light detector 46, respectively. The light source 44 andlight detector 46 are connected to a computing device 48 that performsthe various computational steps in the metrology process. Although FIG.14 illustrates the light source 44 and a light detector 46 as positionedoutside the factory interface 100, these components could be locatedinside the factory interface 100.

A bracket 502 secured to a wall of the factory interface 100 can holdthe trunk 55 of the optical fiber 54 in a fixed position inside thefactory interface 100. The robot 130 can be controlled to sweep thesubstrate at a working distance of two to thirty-five millimeters fromthe end 504 of the optical fiber.

The factory interface 100 can also include a pre-aligner 510 to positionthe substrate in a known rotational position. The pre-aligner 510includes a rotatable support 512, such as a pedestal, possibly with avacuum or electrostatic chuck, an edge support ring, or support pins,onto which the substrate can be placed. In addition, the pre-aligner 510includes a notch detection system, such as an optical interrupter sensor520, to sense when the substrate notch is at a specific angularposition. During creation of a library, the reference substrate isplaced by the robot 130 on the support 512, the support 512 rotates sothat the sensor 520 detects the substrate notch, and rotates to placethe substrate notch in a predetermined angular orientation. Then therobot 130 retrieves the substrate from the support 512. Thus, substrateswhich might be in an uncertain angular position, e.g., after a polishingoperation, have a known orientation when scanned by the in-linespectrographic monitoring system 500, thus permitting accuratedetermination of the x-y (or r-θ) position of the measurements on thesubstrate. Because the position of the spectra measurements is knownwith higher accuracy, the reliability of the association of spectrameasurements with substrate characteristics is improved.

The substrate processing system 8 can operate in two modes: an initiallibrary creation mode and a later in-line monitoring mode. In thelibrary creation mode the substrate processing system can generate alibrary for a particular type of substrate, e.g., a particular patternand a particular metal or dielectric level in the fabrication process.In general, a separate library is created for each different metal ordielectric level in the fabrication process for each different pattern.In the in-line monitoring mode, the substrate processing system 8 usesthe previously generated library to perform quickly determine thecharacteristics of substrates undergoing processing based on themeasured spectrographic data.

Library generation occurs generally as discussed above with respect toFIG. 3. A reference substrate with a particular pattern and at aparticular point in the fabrication process is measured using aconventional metrology system that provides very precise positioning ofa sensor to well-defined locations on the substrate, e.g., a Nova orNanometrics optical metrology system. The measurements can be madebefore or after a polishing step in the fabrication process. At leastone characteristic of a reference substrate, e.g., layer thickness, ismeasured at multiple well-defined locations on the reference substrate,and the measured characteristic and the measurement location are stored,e.g., in a first data structure. The metrology system can be an in-linesystem within the processing system 8, or a stand alone system. However,one potential advantage of using the in-line spectrographic monitoringsystem described herein is that the processing system 8 need not includethe conventional metrology system. In particular, because theconventional metrology system is needed only for accurate substratecharacteristic measurements during library generation (rather thanduring production), a single stand alone metrology system should be ableprovide the necessary measurements for library generation for multipleprocessing systems 8.

The reference substrate 10 a is placed into a cassette 112 and extractedfrom the cassette into the factory interface by the robot 130. The robot130 moves the reference substrate to engage the pre-aligner so that theposition of the substrate can be precisely identified. Then thereference substrate is held by the robot and moved past the opticalprobe. A sequence of spectrographic measurements are generated using thein-line metrology system 500, the position of each spectrographicmeasurement on substrate is determined, and the spectra and measurementlocations are stored, e.g., in a second data structure.

For gathering of spectrographic data for library generation, anexemplary path 520 of the optical probe 504 across a reference substrate10 a having a notch 11 is illustrated in FIG. 15. The path 520 caninclude several arcs 522 that pass along the substrate edge, e.g.,within 8 mm, e.g., within 5 mm, of the substrate edge, to ensure that asignificant number of measurements are obtained near the substrate edge.

Once both spectrographic data and characteristic measurements atwell-defined locations are obtained, the library can then be generatedby associating each spectrographic measurement from the first datastructure with a characteristic measurement from the first datastructure at a nearby well-defined location. The spectra can be measuredat different locations on the reference substrate 10 by the in-linespectrographic monitoring system 500 before or after the substratecharacteristic is measured by the metrology system.

Returning to FIGS. 13 and 14, during processing of device substrates,e.g., in a normal polishing operation, an unpolished substrate isretrieved by the factory interface robot 130 from one of the cassettes112. The factory interface robot 130 “picks” the substrate, e.g., byvacuum suction, and transports the unpolished substrate at relativelyhigh speed past the optical probe of the in-line spectrographicmonitoring system 500 in the factory interface. Thus, the robot 130 actsas the stage to hold the substrate during the measurement process. Thein-line spectrographic monitoring system 500 measures spectra for asequence of points across the substrate as the substrate is scanned, anda layer thickness measurement is generated for at least some of themeasured points. These pre-polish layer thickness measurements can beused to adjust the polishing process parameters for the substrate.

The robot 130 then transports the substrate through the entry port 120to the staging section 176. There, the substrate is placed in either thepass-through support 180 or the indexible buffer 182. The wet robot 140then extracts the substrate 10 from the staging section 176 and placesthe substrate 10 into the transfer station 152 of the polishingapparatus 20. From the transfer station 152, the substrate 10 is carriedto one or more polishing stations 150 to undergo chemical mechanicalpolishing. After polishing, the wet robot 140 transports the substrate10 from the transfer station 152 to the walking beam 184 in the cleaner120. The walking beam 184 transports the substrate through the cleanersection 178 of the cleaner 120. While the substrate 10 is transportedthrough the cleaner section 178, slurry and other contaminants that haveaccumulated on substrate surface during polishing are removed.

The factory interface robot 130 removes the substrate 10 from thecleaner 120 through the exit port 122, and transports the polishedsubstrate at relatively high speed past the optical probe of the in-linespectrographic monitoring system 500 in the factory interface 100.Again, the in-line spectrographic monitoring system 500 measures spectrafor a sequence of points across the substrate as the substrate isscanned, and a layer thickness measurement is generated for at leastsome of the measured points. These post-polish layer thicknessmeasurements can be used to adjust the polishing process parameters fora subsequent substrate. Finally, the factory interface robot 130 returnsthe substrate 10 to one of the cassettes 112.

For gathering of spectrographic data during device substrate processingfor control of polishing parameters, an exemplary path 530 of theoptical probe 504 across a device substrate 10 b having a notch 11 isillustrated in FIG. 16. In some implementations, the path 530 describesa “figure eight” shape on the substrate. The path 530 can includeseveral arcs 532 that pass along the substrate edge, e.g., within 8 mm,e.g., within 5 mm, of the substrate edge, to ensure that a significantnumber of measurements are obtained near the substrate edge.

The robot 130 can move the substrate at a fairly high speed across thesubstrate. For example, the robot could move a 300 mm diameter substrateto cause the optical probe to trace the path shown in FIG. 16 in aboutthree to seven seconds, e.g., about six seconds. The detector 46 canhave a sampling rate of about 130 to 150 samples per second, e.g., 142samples per second (the light source 44 can flash on for eachspectrographic measurement). Thus, assuming that path 530 is traced overabout 6 seconds, about 850 spectra can be measured along the path. Dueto the high speed of the in-line measurement, e.g., a velocity of about150-350 mm/sec during many measurements, during production each andevery substrate can undergo both pre-polish and post-polish measurementwithout impacting substrate throughput (for throughput <85 wafer perhour). Thus, for each substrate, thickness measurements at a variety ofradial positions on the substrate can be used to control processingconditions for that substrate or for a subsequent substrate.

Optionally, the in-line spectrographic metrology system could be housedin a separate module 160 connected to the factory interface module 100.For example, one of the side walls 104 or 106 (side wall 106 in theimplementation shown in FIG. 12) mates with a wall 161 of the metrologymodule 160 and shares an access port 124. The side wall 104 and themonitoring system wall 161 may be combined into one structure, and theremay be additional ports from the factory interface module 100 to themetrology module 160. The metrology module 160 could include a separaterobot for the substrate, or the factory interface robot 130 couldmanipulate the substrate, to cause the substrate to be scanned past thespectrographic probe.

The subject matter described herein contemplates a comprehensivethin-film metrology and polishing system, which combines measurements ofpatterned wafers irrespective of locations of the measurements. Itoffers both real-time, in-line measurements (i.e. performed within asemiconductor fabrication tool) and also rapid multi-point (i.e.mapping) at-line measurements of film thickness, composition, andelectronic properties. The present concepts can be applied broadly tomany of the critical electronic materials that are processed insemiconductor fabrication tools, including polysilicon, silicon dioxide,silicon nitride, and other dielectrics.

Implementations and all of the functional operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structural meansdisclosed in this specification and structural equivalents thereof, orin combinations of them. Implementations described herein can beimplemented as one or more computer program products, i.e., one or morecomputer programs tangibly embodied in an information carrier, e.g., ina machine readable storage device or in a propagated signal, forexecution by, or to control the operation of, data processing apparatus,e.g., a programmable processor, a computer, or multiple processors orcomputers. A computer program (also known as a program, software,software application, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, and it can bedeployed in any form, including as a stand alone program or as a module,component, subroutine, or other unit suitable for use in a computingenvironment. A computer program does not necessarily correspond to afile. A program can be stored in a portion of a file that holds otherprograms or data, in a single file dedicated to the program in question,or in multiple coordinated files (e.g., files that store one or moremodules, sub programs, or portions of code). A computer program can bedeployed to be executed on one computer or on multiple computers at onesite or distributed across multiple sites and interconnected by acommunication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

The above described polishing apparatus and methods can be applied in avariety of polishing systems. Either the polishing pad, or the carrierhead, or both can move to provide relative motion between the polishingsurface and the wafer. For example, the platen may orbit rather thanrotate. The polishing pad can be a circular (or some other shape) padsecured to the platen. Some aspects of the endpoint detection system maybe applicable to linear polishing systems (e.g., where the polishing padis a continuous or a reel-to-reel belt that moves linearly). Thepolishing layer can be a standard (for example, polyurethane with orwithout fillers) polishing material, a soft material, or afixed-abrasive material. Terms of relative positioning are used; itshould be understood that the polishing surface and wafer can be held ina vertical orientation or some other orientations.

Particular implementations have been described. Other implementationsare within the scope of the following claims. For example, the actionsrecited in the claims can be performed in a different order and stillachieve desirable results.

1. A method of generating a library for use in processing productwafers, the method comprising: measuring substrate layer thickness at afirst well-defined point and a second well-defined point of a referencesubstrate with a first metrology system; measuring a spectrum at ameasurement point of the reference substrate with a second monitoringsystem other than the first metrology system; determining a closerwell-defined point of the first well-defined point and the secondwell-defined point to the measurement point; and storing an associationof the spectrum with the substrate layer thickness of the closerwell-defined point.
 2. The method of claim 1, wherein substrate layerthickness is measured prior to measuring the spectrum.
 3. The method ofclaim 1, wherein measuring the spectrum comprises scanning a sensoracross the reference substrate and measuring a plurality of spectra at aplurality of measurement points including the spectrum at themeasurement point.
 4. The method of claim 3, further comprisingmeasuring substrate layer thicknesses of the reference substrate at aplurality of well-defined points with the first metrology system, theplurality of well-defined points including the first well-defined pointand the second well-defined point.
 5. The method of claim 4, furthercomprising determining a closest well-defined point of the plurality ofwell-defined points for each of the plurality of measurement points. 6.The method of claim 5, further comprising, for each spectrum from theplurality of spectra, storing an association of the spectrum from theplurality of spectra with a substrate layer thickness of the closestwell-defined point.
 7. The method of claim 6, further comprising:scanning a product substrate other than the reference substrate with anoptical monitoring system to generate a measured spectrum of the productsubstrate; determining a best matching spectrum from the plurality ofspectra to the measured spectrum of the product substrate; anddetermining a substrate layer thickness associated with the bestmatching spectrum.
 8. The method of claim 7, wherein second monitoringsystem is the optical monitoring system.
 9. The method of claim 1,wherein the first well defined point and the second well defined pointare in different dies on the reference substrate.
 10. The method ofclaim 9, wherein the first well defined point and the second welldefined point are at the same relative position within the differentdies.
 11. A computer program product, tangibly stored on machinereadable storage device, for generating a library for use in processingproduct wafers, the product comprising instructions operable to cause aprocessor to: cause substrate layer thickness to be measured at a firstwell-defined point and a second well-defined point of a referencesubstrate with a first metrology system; cause a spectrum to be measuredat a first measurement point of the reference substrate with a secondmonitoring system other than the first metrology system; determine acloser well-defined point of the first well-defined point and the secondwell-defined point to the first measurement point; and store anassociation of the spectrum with the substrate layer thickness of thecloser well-defined point.
 12. A method of generating a library for usein processing product wafers, the method comprising: measuring a firstvalue of a substrate layer characteristic at a first well-defined pointof a reference substrate with a first metrology system and measuring asecond value of the substrate layer characteristic at a secondwell-defined point of the reference substrate with the first metrologysystem; measuring a spectrum at a measurement point of the referencesubstrate with a second monitoring system other than the first metrologysystem; determining a first distance from the measurement point to thefirst well-defined point and a second distance from the measurementpoint to the second well-defined point; calculating a third value fromthe first value, the second value, the first distance and the seconddistance; and storing an association of the spectrum with the thirdvalue.
 13. The method of claim 12, wherein the substrate layercharacteristic comprises a layer thickness.
 14. The method of claim 12,wherein calculating the third value comprises calculating a weightedaverage of the first value and the second value with weighting based onthe first distance and the second distance.
 15. The method of claim 12,further comprising measuring the substrate layer characteristic of thereference substrate at a plurality of well-defined points with the firstmetrology system to generate a plurality of values, the plurality ofwell-defined points including the first well-defined point and thesecond well-defined point.
 16. The method of claim 14, wherein the firstwell-defined point and the second well defined point are the closestwell-defined points of the plurality of well-defined points to themeasurement point.
 17. The method of claim 15, wherein measuring thespectrum comprises scanning a sensor across the reference substrate andmeasuring a plurality of spectra at a plurality of measurement pointsincluding the spectrum at the measurement point.
 18. The method of claim16, further comprising, for each spectrum from the plurality of spectra,determining distances from the measurement point to two of the pluralityof well-defined points and calculating a value from values of thesubstrate layer characteristic at the two of the plurality ofwell-defined points and the distances.
 19. The method of claim 17,further comprising: scanning a product substrate other than thereference substrate with an optical monitoring system to generate ameasured spectrum of the product substrate; determining a best matchingspectrum from the plurality of spectra to the measured spectrum of theproduct substrate; and determining the value of the substrate layercharacteristic associated with the best matching spectrum.
 20. Themethod of claim 12, wherein the first well defined point and the secondwell defined point are in different dies on the reference substrate. 21.The method of claim 19, wherein the first well defined point and thesecond well defined point are at the same relative position within thedifferent dies.
 22. A computer program product, tangibly stored onmachine readable storage device, for generating a library for use inprocessing product wafers, the product comprising instructions operableto cause a processor to: cause a first value of a substrate layercharacteristic to be measured at a first well-defined point of areference substrate with a first metrology system and cause a secondvalue of the substrate layer characteristic to be measured at a secondwell-defined point of the reference substrate with the first metrologysystem; cause a spectrum to be measured at a measurement point of thereference substrate with a second monitoring system other than the firstmetrology system; determine a first distance from the measurement pointto the first well-defined point and a second distance from themeasurement point to the second well-defined point; calculate a thirdvalue from the first value, the second value, the first distance and thesecond distance; and store an association of the spectrum with the thirdvalue.